The Democrats' long battle to pass financial reform is coming to a head Monday, with Senate Majority Leader Harry Reid (D-NV) planning a test vote as early as this evening. The Democratic bill would introduce tough new rules to protect consumers, regulate derivative markets, and beef up bank capital requirements. But Republicans say it is too expensive, and could limit Wall Street's profitability — they are demanding concessions under threat of filibuster. What will happen next? (Watch The Week's Sunday Talk Show Briefing on the Senate's hopes for financial regulation.) Here are three theories:

The GOP filibustersAll 41 Senate Republicans would need to come together to hold a fillibuster. Bob Corker (R-TN) told "ABC's This Week" that this outcome was "very likely." Indeed, neither side seems that unhappy about a filibuster, says Brady Dennis at The Washington Post. "Democrats could say the GOP is standing in the way of reform, and Republicans could say the bill's flaws would cause the economy more harm than good." But Wall Street remains very unpopular with the American public, and some variation of this bill would likely be revived within a few days, after more negotiations.

A last minute compromiseIf the GOP opposition holds fast and the Democrats want to avoid a filibuster, they could cede ground on a number of contentious elements in the bill (including dropping plans for a $50 billion bank "bail-out fund" and the so-called "Volcker Rule" imposing strict limits on banks' rights to trade financial derivatives). The Democrats have "appeared willing to jettison" the $50 billion fund to achieve a bipartisan agreement, says Jim Kuhnhenn at AP, but need the derivatives reform to satisfy doubters within their own party.

A lone Republican rebel changes everythingIf just one Republican can be persuaded to break ranks, Democrats will be allowed to bring their bill to the Senate floor for debate (and, likely, approval by a comfortable margin). Reid has been targeting GOP senators Olympia Snowe (R-ME) and Charles Grassley (R-IA) to cross the aisle, though each is reportedly asking for substantive changes to the bill. Most other moderate Republicans — including Scott Brown (R-MA) — have said they will join the filibuster.